The use of bitumen (asphalt) compositions in preparing aggregate compositions (including, but not just limited to, bitumen and rock) useful as road paving material is complicated by at least three factors, each of which imposes a serious challenge to providing an acceptable product. First, the bitumen compositions must meet certain performance criteria or specifications in order to be considered useful for road paving. For example, to ensure acceptable performance, state and federal agencies issue specifications for various bitumen applications including specifications for use as road pavement. Current Federal Highway Administration specifications require a bitumen (asphalt) product to meet defined parameters relating to properties such as viscosity, stiffness, penetration, toughness, tenacity and ductility. Each of these parameters define a critical feature of the bitumen composition, and compositions failing to meet one or more of these parameters will render that composition unacceptable for use as road pavement material.
Conventional bitumen compositions frequently cannot meet all of the requirements of a particular specification simultaneously and, if these specifications are not met, damage to the resulting road can occur, including, but not necessarily limited to, permanent deformation, thermally induced cracking and flexural fatigue. This damage greatly reduces the effective life of paved roads.
In this regard, it has long been recognized that the properties of conventional bitumen compositions can be modified by the addition of other substances, such as polymers. A wide variety of polymers have been used as additives in bitumen compositions. For example, copolymers derived from styrene and conjugated dienes, such as butadiene or isoprene, are particularly useful, since these copolymers have good solubility in bitumen compositions and the resulting modified-bitumen compositions have good rheological properties.
It is also known that the stability of polymer-bitumen compositions can be increased by the addition of crosslinking agents (vulcanizing agents) such as sulfur, frequently in the form of elemental sulfur. It is believed that the sulfur chemically couples the polymer and the bitumen through sulfide and/or polysulfide bonds. The addition of extraneous sulfur may be helpful to produce improved stability, even though bitumens naturally contain varying amounts of native sulfur.
Thus, there are known processes for preparing a bitumen-polymer composition consisting of mixing a bitumen, at temperatures of about 266-446° F. (130-230° C.), with 2 to 20% by weight of a block or random copolymer, having an average molecular weight between 30,000 and 300,000. The resulting mixture is stirred for at least two hours, and then 0.1 to 3% by weight of sulfur relative to the bitumen is added and the mixture agitated for at least 20 minutes. The quantity of added sulfur can be from about 0.1 to 1.5% by weight with respect to the bitumen. The resulting bitumen-polymer composition is used for road-coating, industrial coating, or other industrial applications.
Similarly, there are also known asphalt (bitumen) polymer compositions obtained by hot-blending asphalt with from about 0.1 to 1.5% by weight of elemental sulfur and 1 to 7% by weight of a natural or synthetic rubber, which can be a linear butadiene/styrene copolymer. A process is additionally known for preparing a rubber-modified bitumen by blending rubber, either natural or synthetic, such as styrene/butadiene rubber, with bitumen at 280-400° F. (138-204° C.), in an amount up to 10% by weight based on the bitumen, then adjusting the temperature to 257-320° F. (125-160° C.), and intimately blending into the mix an amount of sulfur such that the weight ratio of sulfur to rubber is between 0.01 and 0.9. A catalytic quantity of a vulcanization-accelerator is also added to effect vulcanization. A critical nature of the sulfur to rubber ratio is sometimes reported, for instance that weight ratios of sulfur to rubber of less than 0.01 gives modified bitumen of inferior quality.
A second factor complicating the use of bitumen compositions concerns the viscosity stability of such compositions under storage conditions. In this regard, bitumen compositions are frequently stored for up to 7 days or more before being used and, in some cases, the viscosity of the composition can increase so much that the bitumen composition is unusable for its intended purpose. On the other hand, a storage stable bitumen composition would provide for only minimal viscosity increases and, accordingly, after storage it can still be employed for its intended purpose.
Asphaltic concrete, typically including asphalt and aggregate, asphalt compositions for resurfacing asphaltic concrete, and similar asphalt compositions must exhibit a certain number of specific mechanical properties to enable their use in various fields of application, especially when the asphalts are used as binders for superficial coats (road surfacing), as asphalt emulsions, or in industrial applications. (The term “asphalt” is used herein interchangeably with “bitumen.” Asphaltic concrete is asphalt used as a binder with appropriate aggregate added, typically for use in roadways.) The use of asphalt or asphalt emulsion binders either in maintenance facings as a surface coat or as a very thin bituminous mix, or as a thicker structural layer of bituminous mix in asphaltic concrete, is enhanced if these binders possess the requisite properties such as desirable levels of elasticity and plasticity.
As noted, various polymers have been added to asphalts to improve physical and mechanical performance properties. Polymer-modified asphalts (PMAs) are routinely used in the road construction/maintenance and roofing industries. Conventional asphalts often do not retain sufficient elasticity in use and, also, exhibit a plasticity range that is too narrow for use in many modern applications such as road construction. It is known that the characteristics of road asphalts and the like can be greatly improved by incorporating into them an elastomeric-type polymer which may be one such as butyl, polybutadiene, polyisoprene or polyisobutene rubber, ethylene/vinyl acetate copolymer, polyacrylate, polymethacrylate, polychloroprene, polynorbornene, ethylene/propylene/diene (EPDM) terpolymer and advantageously a random or block copolymer of styrene and a conjugated diene. The modified asphalts thus obtained commonly are referred to variously as bitumen/polymer binders or asphalt/polymer mixes. Modified asphalts and asphalt emulsions typically are produced utilizing styrene/butadiene based polymers, and typically have raised softening point, increased viscoelasticity, enhanced force under strain, enhanced strain recovery, and improved low temperature strain characteristics as compared with non-modified asphalts and asphalt emulsions.
The bituminous binders, even of the bitumen/polymer type, which are presently employed in road applications often do not have the optimum characteristics at low enough polymer concentrations to consistently meet the increasing structural and workability requirements imposed on roadway structures and their construction. In order to achieve a given level of modified asphalt performance, various polymers are added at some prescribed concentration.
Current practice is to add the desired level of a single polymer, sometimes along with a reactant that promotes cross-linking of the polymer molecules until the desired asphalt properties are met. This reactant typically is sulfur in a form suitable for reacting.
However, the cost of the polymer adds significantly to the overall cost of the resulting asphalt/polymer mix. Thus, cost factors weigh in the ability to meet the above criteria for various asphalt mixes. In addition, at increasing levels of polymer concentration, the working viscosity of the asphalt mix becomes excessively great and separation of the asphalt and polymer may occur.
It is common in the preparation of polymer-modified asphalts to include activators and accelerators to make the crosslinking reaction proceed faster. Zinc oxide (ZnO) is a conventional activator, and mercaptobenzothiazole (MBT) is a conventional accelerator. ZnO is also sometimes used to control the tendency of the polymer to gel. The zinc salt of mercaptobenzothiazole (ZMBT) combines features of both of these conventional additives.
As can be seen from the above, methods are known to improve the mixing of asphalt and polymer compositions. The needed elements for the commercial success of any such process include keeping the process as simple as possible, reducing the cost of the ingredients, and utilizing available asphalt cuts from a refinery without having to blend in more valuable fractions. In addition, the resulting asphalt composition must meet the above-mentioned governmental physical properties and environmental concerns. Thus, it is a goal of the industry to maintain or reduce the cost of the polymers and crosslinking agents added to the asphalt without sacrificing any of the other elements and improving the properties of the asphalt and polymer compositions as much as possible.